fix(scan): emit a client update for ROMs that are no longer missing

[gantoine]

Description
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Reported on Discord: re-adding a game's file only clears the "missing" badge when you quick/hash scan that game, never when you scan the platform. It looks like a platform scan skips missing games.

The database was actually being repaired correctly all along. _identify_platform syncs missing_from_fs against the filesystem before the identify loop, and bulk_mark_present clears it again for skipped entries. What was missing is the client-facing signal: a ROM that already exists is skipped by should_scan_rom on a quick scan, and only ROMs that go through _identify_rom emit scan:scanning_rom. So an open gallery kept showing the stale badge until a refetch, while a single-game scan (which forces the ROM through the scan path via roms_ids) visibly fixed it.

Changes:

• get_missing_rom_ids(platform_id) in roms_handler returns the ids currently flagged missing.
• _identify_platform snapshots those ids before the pre-loop mark_missing_roms sync clears them. Without the snapshot there's nothing left to detect: by the time the batch loop loads the ROM, the flag is already False.
• After bulk_mark_present, each restored entry is logged and re-emitted over scan:scanning_rom, reloaded via get_rom so the schema has its relationships hydrated (the scan-loop lookup deliberately only eager-loads platform).

The frontend already merges scan:scanning_rom into the gallery and stores, so the badge now clears live during a platform scan and the restored ROM shows up in the scan progress list.

Deliberately out of scope: restored ROMs still don't get their RomFile rows and hashes rebuilt on a quick scan. Doing that would rehash the whole library in the common "NAS unmounted, then remounted" case. A complete or hash scan still rebuilds them.

Checklist
^{Please check all that apply.}

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Tests: TestIdentifyPlatformEmitsRestoredRoms (emits for a restored ROM, stays silent for one that was already present) and test_get_missing_rom_ids. tests/endpoints/sockets/test_scan.py 49 passed, the missing/bulk-mark DB handler tests 17 passed, trunk fmt && trunk check clean.

Screenshots (if applicable)

N/A, backend only.

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AI assistance disclosure: this change was written with AI assistance (Claude Code). The investigation, the patch, and the tests were AI-generated, then reviewed and verified locally by me.

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[greptile-apps]

Greptile Summary

Updates platform scans to notify connected clients when previously missing ROMs reappear.

• Adds a platform-scoped database query for missing ROM IDs.
• Snapshots missing IDs before filesystem synchronization and emits hydrated restored ROMs through scan:scanning_rom.
• Adds socket-scan and database-handler tests for restored-ROM notifications and missing-ID filtering.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The restored-ROM path snapshots state before synchronization, updates persisted presence before reloading, and emits the same hydrated payload shape already used by existing scan notifications.

Important Files Changed

Filename	Overview
backend/endpoints/sockets/scan.py	Correctly snapshots missing ROM IDs before synchronization and emits restored skipped ROMs using the established hydrated socket payload path.
backend/handler/database/roms_handler.py	Adds a platform-scoped, dialect-portable query returning IDs currently marked missing.
backend/tests/endpoints/sockets/test_scan.py	Covers notification behavior for restored ROMs and verifies already-present ROMs remain silent.
backend/tests/handler/test_db_handler.py	Verifies missing-ROM ID filtering is limited to flagged ROMs on the requested platform.

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